Frequently, in the production of thermoplastic containers, such as polyethylene terephthalate (PET) bottles, preforms are first heated and subsequently molded (e.g., via injection molding or stretching blow molding) to form containers of the desired shapes. The heating operation is usually performed with infrared radiation or near-infrared radiation, which is associated with low energy efficiency, typically at approximately 10-15%. Hence, duration for infrared heating can be long, usually on the order of minutes. In contrast, heating preforms with microwave radiation can have higher energy efficiency (e.g., at approximately 30-50%), which can dramatically reduce energy cost in comparison to the use of infrared heating.
Microwave-based heating apparatuses for plastic preforms are known, such as the apparatuses described in U.S. Pat. Nos. 6,952,949, 7,163,655, 8,231,823, 8,517,711 and 8,629,379. These microwave devices, however, cannot heat a preform substantially uniformly without mechanically moving a preform linearly and/or rotationally during microwave heating. A disadvantage associated with such a device is that by heating a portion of the preform at a time, the production throughput is greatly reduced. In addition, the mechanical movement increases the complexity and cost of the preform heating devices. Furthermore, no existing devices can provide relatively uniform and efficient heating of a preform with non-uniform thickness or geometry, which exists in almost all preforms.